Measuring the concentration of nitrogen monoxide (NO) in respired gas is an important means for optimizing the treatment of asthmatic conditions. One promising technology for the detection of nitrogen monoxide is an NO2 sensor based on suspended gate FET technology. The structure of these sensors is known, for example, from the documents DE 19 814 857 or DE 19 956 744. Field effect transistor-based gas sensors have the advantage of simple production by using standard processes (CMOS) as well as a low energy demand in operation. Another advantage of these sensors is that they comprise a sensitive layer for the gas detection. The material of the sensitive layer may in this case be selected almost freely, and it is therefore possible to produce a range of different gas sensors on the same basis.
For the measurement of nitrogen dioxide, a layer of a porphyrin dye or phthalocyanine, in particular copper phthalocyanine, for example, has been found to be particularly promising. Nitrogen dioxide is adsorbed on this layer, and leads to a potential change and therefore to a measurable signal. If the nitrogen dioxide disappears from the ambient air, then the nitrogen dioxide bound on the surface of the copper phthalocyanine layer is desorbed and the signal falls off. One difficulty in this case is that the desorption of the nitrogen dioxide takes place very slowly. For instance, the t90 time at room temperature is more than 1 h.